-LRB- CNN -RRB- -- There are fears of Russians and it is October , but it 's not a Tom Clancy novel . It is a case of international naval intrigue off the Swedish coast that brings back memories of the Cold War .

The Swedish military on Monday intensified a search in the ocean off Stockholm for an underwater mystery vessel , but stopped short of calling it a submarine . Civilian vessels were ordered to stay at least six miles -LRB- about 10 kilometers -RRB- away from the Swedish warship conducting the search , the English-language website The Local reported .

The search began Thursday after Swedish intelligence picked up an emergency radio call in Russian , reported The Local , citing the Swedish newspaper Svenska Dagbladet .

The radio transmissions were being sent to the Russian enclave of Kaliningrad , 330 miles -LRB- 530 kilometers -RRB- south of Stockholm on the Baltic 's southern shore , according to The Local report .

There were also reports that a foreign vessel was spotted in the waters near Stockholm .

Russia on Sunday denied it has any vessel in Swedish waters . Moscow suggested the vessel may belong to the Netherlands and have been involved in naval exercises off Sweden , according to a report from Russia 's Itar-TASS news agency .

The Dutch were quick to respond , saying a sub involved in the exercises was anchored in Tallinn harbor of NATO-ally Estonia for the weekend , according to a report from Agence France-Presse .

` We have good stamina '

On Monday , the Swedish military vowed to continue searching at least for the next few days , according to The Local .

`` We have good stamina , '' The Local quoted defense spokesman Dag Enander . `` We 're using the sensors we have and are searching both from land and water , '' Enander was quoted as saying .

To confuse matters even more , the Swedish military said Monday that it purposely provided incorrect information on Sunday about where the mystery vessel was sighted to keep the search area secure .

U.S. officials told CNN there are no U.S. submarines or surface ships in the immediate area . U.S. intelligence does n't know of any Russian assets in that area , the officials said .

All the activity begs the question , why would a Russian sub need to snoop around in Swedish waters anyway ?

Johan Wiktorin of the Swedish Royal Academy of War Sciences gave three possibilities to The Local :

`` They could be mapping the waters in order to be able to navigate them in the event of hostilities , '' Wiktorin is quoted as saying .

`` They could also be installing equipment , like sensors , that could track or get an observation of our units in that area , '' he goes on . `` Or they could also possibly reconnoiter our -LRB- defense -RRB- systems . ''

Russian military around the world

Whether there is a Russian sub in Swedish waters or not , there is no question that Russian forces have been active in the Baltic and around the globe this year . In fact , the Swedish military said Sunday that it has monitored suspicious activity for several years , according to another report in The Local .

In July , a U.S. Air Force reconnaissance plane fled into Swedish airspace after the Russians took the unusual action of beginning to track it with land-based radar . The Russians then sent at least one fighter jet into the sky to intercept the U.S. aircraft .

And on April 23 , a Russian Su-27 Flanker fighter jet buzzed within 100 feet of the nose of a U.S. Air Force RC-135U reconnaissance plane over the Sea of Okhotsk between Russia and Japan , a Defense Department official said .

Also on April 23 , Dutch fighter jets scrambled to intercept a pair of Russian Tu-95 Bear bombers that entered a half-mile into Netherlands airspace .

On June 4 , according to U.S. defense officials , four long-range Russian Tu-95 bombers , accompanied by an aerial refueling tanker , flew into the U.S. Air Defense Identification Zone , an area extending 200 miles -LRB- 320 kilometers -RRB- from the North American coast , off Alaska , where they were intercepted by U.S. F-22 fighter jets .

Two of the Russian bombers peeled off and headed west , while the other two flew south and were identified by U.S. F-15 fighters within 50 miles of the California coast .

And last month , two Alaskan-based U.S. Air Force F-22 fighter jets intercepted two Russian IL-78 refueling tankers , two Russian MiG-31 fighter jets and two Russian Bear long-range bombers , according to Capt. Jeff Davis of the North American Aerospace Defense Command .

More than six hours later , two Canadian CF-18 fighter jets intercepted two Russian bombers in the Beaufort Sea , Davis said . Those Russian planes came within about 40 nautical miles of the Canadian coastline , he said .

Earlier this year , a top U.S. Air Force general said Russia was stepping up its military activities in the Asia-Pacific region as tensions increased over Ukraine and Russia 's move into Crimea .

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Emergency radio call picked up by Swedish military , report says

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Radio transmissions sent to Russian enclave on Baltic Sea , report says

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Russia denies any vessel in Swedish waters

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Russian military has been active around world this year